In a spot welding machine, the top of an electrode tip (hereinafter top electrode tip), when electrified, is exposed to a high temperature under a high pressure. The top electrode tip of the spot welding machine is thus deformed each time the spot welding machine is electrified, which gradually increases the diameter of a flat portion of the top electrode tip. In the spot welding (resistance welding), on condition that status such as a welding current, etc. is fixed, if the diameter of the flat portion of the top electrode tip is increased, current density of the welding current applied to a jointed portion of work is decreased, which makes difficult to form a well-conditioned nugget. Accordingly, the strength of the jointed portion of the work is deteriorated, which eventually lowers product reliability. Therefore, in the spot welding (resistance welding) machine, each time the top electrode tip is electrified, an amount of the welding current is gradually increased, keeping constant the current density of the welding current applied to the jointed portion of the work. This kind of the spot welding machine provided with a step-up function has been generally used. With the spot welding machine, the well-conditioned nugget is continuously obtainable.
However, in the spot welding (resistance welding), a surface treatment such as galvanization, etc. is often applied to the work. In case the spot welding is continuously applied to the work under the surface treatment, a surface treatment material such as zinc melted may adhere to the top electrode tip due to an exoergic heat during the electrification. This may cause adhesion between the work and the top electrode tip. Thus, the top electrode tip is dressed periodically, for example, in every 50 welding, so that the adhesion between the work and the top electrode tip is prevented. In general, in a welding robot provided with a spot welding gun, each time a given number of welding is completed, the welding gun is made to set at an electrode tip dressing position, so that the top electrode tip is dressed by means of an electrode tip dressing apparatus. This type of the dressing is conducted by: driving rotatively a dressing tool of the electrode tip dressing apparatus (see, for example, Patent Document 1) with an electric motor; pressing a pair of the top electrode tip to the dressing tool; and cutting the top electrode tip with the dressing tool.
Now, in the electrode tip dressing apparatus, for cutting and finishing the top electrode tip in a good manner, the top electrode tip needs to be cut for at least 0.2 mm. Further, in order to ensure a certain welding quality in a spot welding (resistance welding), it is recommendable to dress (cut) the top electrode tip in every single work. Accordingly, to obtain the welding quality, if 40 works are conducted, or the top electrode tip is cut for 8 mm, the electrode tip needs to be exchanged. This not only notably lowers productivity but also increases cost. Thus, as shown in FIG. 9, there has been an electrode tip dressing apparatus 1 wherein dressing tools 2 are exchanged from a cutting type to a plastic-deformation type. In this electrode tip dressing apparatus 1, while contacting top of an electrode tip (hereinafter top electrode tip) to the dressing tools 2, the dressing tools 2 are rotated around the electrode tip, whereby the top electrode tip is plastically deformed and dressed to a given form.
However, in this type of the electrode tip dressing apparatus 1, even if a contact pressure between the top electrode tip and a dressing portion 4 of each of the dressing tools 2 is made uniform, there occurs a different circumferential speed between a rotational center of the dressing tools 2 and an outer side thereof. Since a relative traveling speed to the top electrode tip is different between the rotational center of the dressing tools 2 and the outer side thereof, the dressing done with the dressing tools 2 is not constant. In the conventional electrode tip dressing tool, the outer rotational side of the dressing tools 2 is subject to an excess amount of work, thus increasing a rotational driving-force of the dressing tools 2. Therefore, a larger electric motor to drive the dressing tools 2 is required, thereby making the electrode tip dressing apparatus 1 eventually larger.
Patent Document 1: Japanese Patent Application Laid-open No. H11-77331